N'DJAMENA/YAOUNDE, March 12 (Reuters) - Much of the Chadian force fighting Boko Haram in Nigeria has withdrawn to Cameroon and is preparing to deploy further south, Chadian soldiers and a Cameroonian military official said on Thursday, signalling the potential start of a new offensive.
Chad's military has spearheaded an operation by Nigeria's neighbours against the Islamist militant group that has killed thousands in northeast Nigeria and, in recent months, mounted increasing cross border raids.
Chadian soldiers, who asked not to be identified, said troops operating in Nigeria had pulled back from their forward base of Gambaru to Fotokol, a town on the Cameroon side of the border.
"We don't know the reason for the withdrawal. We just received the orders," one of them said, adding that the force was now heading south to the border town of Banki.
Banki lies on the main road from Cameroon to the Nigerian town of Bama, where Nigeria's military has been planning an offensive.
A spokesman for the Chadian military was not immediately available for comment. Chadian troops are also battling Boko Haram to the north along the border with Niger.
A Cameroonian military source confirmed Chadian troops had crossed back into Cameroon and were headed south but gave no further details.
Chadian troops last week pushed the furthest they have gone into Nigeria when they freed Dikwa, a town at a major crossroad some 80 km (50 miles) east of Maiduguri, the capital of the northeastern Borno state.
Since then, Nigeria asked Chadian troops to leave and deployed its own troops to the town, Chadian officers said.
Mistrust and rivalries have hamstrung coordination between the regional armies, which are in the process of planning and seeking United Nations backing for a joint 10,000-strong force to defeat Boko Haram.
Chadian officials complain that Nigeria has repeatedly prevented them from advancing despite Chad having scored several victories against the militants, who recently pledged allegiance to Islamic State and aim to carve out a caliphate in Nigeria's northeast.
Nigeria's military denies any lack of cooperation and says it has not been credited enough for its own gains against Boko Haram.
With Nigerian President Goodluck Jonathan seeking reelection at polls on March 28, his opponent Muhammadu Buhari has lambasted his government for relying on Chadian intervention to tackle Boko Haram. (Writing by David Lewis; Editing by Daniel Flynn)